91av

BEAUTY really is in the eye of the beholder. Rather than always chasing the
best-looking members of the opposite sex, some animals prefer mates that the
majority find decidedly unattractive.

“Ugly individuals can sometimes do better than good-looking ones,” says
evolutionary biologist Rob Brooks of the University of New South Wales in
Sydney. If the same holds true for people, then it may be time to stop worrying
about how closely you adhere to the Hollywood ideal of attractiveness.

Studies of how birds, fish and people choose mates have found that members of
the same species tend to find the same things attractive. For instance, peahens
prefer to mate with brightly coloured, long-tailed peacocks, while women prefer
tall men. Those attractive traits are supposed to signal beneficial
characteristics such as an increased chance that any offspring will survive.

But there are problems with the idea that all animals prefer the same
features. For a start, you would expect birds and fish of the same
species—and perhaps people too—to look far more similar than they
do, because evolutionary forces would have moulded them into a super-attractive
archetype.

What’s more, most of us know from experience that one person’s dog is another
person’s sex god—and that makes some evolutionary biologists cautious
about assuming that animals have only one ideal of attractiveness. “People say
‘I don’t like what my mate down the pub likes’. How do you reconcile that with a
universal Hollywood idea of attractiveness?” says Brooks.

To find out, he joined forces with John Endler, an ecological geneticist at
James Cook University in Townsville, to observe how individual female guppies
choose between different males. They found that although all the females liked
males with bright orange spots and large tails, a minority of females also liked
males with black markings.

“It’s a nice result,” says Raoul Mulder, an evolutionary ecologist at the
University of Melbourne. “It explains why certain very rare colour patterns
continue to persist in guppy populations.”

Brooks says that previous studies failed to detect idiosyncratic mate choice
because they looked at what whole populations found attractive on average. “But
you don’t mate with an average, you mate with an individual,” he says. There may
be a good reason that attractiveness is idiosyncratic, Brooks says. What
benefits one individual might not necessarily benefit another.

For example, says Brooks, this may be what happens with immune system
proteins called MHC molecules. The more diverse a selection you inherit from
your parents, the better equipped you are to fight off disease. Men with
different MHCs smell differently, and not only can women detect that difference,
but they also tend to prefer the smell of men whose MHCs complement their own.
This makes sense because any children resulting from such a pairing would have
more diverse MHCs
(91av, 10 February, p 36).

“MHC is a great example of a real genetic benefit to mate choice that isn’t
uniform,” says Brooks.